Phosphorus-bearing molecules in the Galactic Center

Abstract

Phosphorus (P) is one of the essential elements for life due to its central role in biochemical processes. Recent searches have shown that P-bearing molecules (in particular PN and PO) are present in star-forming regions, although their formation routes remain poorly understood. In this Letter, we report observations of PN and PO towards seven molecular clouds located in the Galactic Center, which are characterized by different types of chemistry. PN is detected in five out of seven sources, whose chemistry is thought to be shock-dominated. The two sources with PN non-detections correspond to clouds exposed to intense UV/X-rays/cosmic-ray radiation. PO is detected only towards the cloud G+0.693−0.03, with a PO/PN abundance ratio of ∼1.5. We conclude that P-bearing molecules likely form in shocked gas as a result of dust grain sputtering, while are destroyed by intense UV/X-ray/cosmic ray radiation.

The Chemistry of Phosphorus-bearing Molecules under Energetic Phenomena

“…Millimeter observations of PN and PO in outflows (Lefloch et al 2016) and Galactic center GMCs (Rivilla et al 2018), however, indicate that the PO/PN abundance ratio is always 1, which can be reconciled with our models only if (i) cosmic rays are significantly enhanced or (ii) the chemical network of PO is incomplete. In the following section (Section 4), we explore the possibility that the chemical network of PO lacks key formation reactions.…”

“…In addition, these species have also been detected in the turbulent GMCs in the central molecular zone (Rivilla et al 2018), believed to be affected by widespread low-velocity shocks (Requena-Torres et al 2006) and likely under the influence of enhanced cosmicray ionization rates (Goto et al 2014;Harada et al 2015).…”

“…In this section, we compare our modeling results to the abundances of PN and PO measured toward low-mass/ high-mass starless/pre-stellar cores (Turner et al 1990;Mininni et al 2018), massive hot cores (Turner et al 1990;Rivilla et al 2016), GMCs in the Galactic center (Rivilla et al 2018), and molecular outflows (Yamaguchi et al 2011;Lefloch et al 2016). …”

“…In all cases, PO seems to be slightly more abundant than PN (by factors of 2-3; see, e.g., Rivilla et al 2016), 3 and the PO/PN ratio can potentially be used as a length indicator of the pre-stellar collapse phase (see Aota & Aikawa 2012;Lefloch et al 2016). The analysis of the P-bearing content of the giant molecular clouds (GMCs) in the Galactic center shows that PN and PO can also be detected in molecular clouds subject to extreme conditions, such as those found in galactic nuclei (Rivilla et al 2018).…”

“…While Yamaguchi et al (2011) and Lefloch et al (2016) proposed that P-bearing species are produced in shocked gas after the sputtering of dust grains, Rivilla et al (2016) suggested that P-bearing molecules are formed during the cold collapse phase and subsequently frozen onto dust grains (Caux et al 2011;Fontani et al 2016;Rivilla et al 2016). The detection of PN and PO in the Galactic center poses even more unknowns to the chemistry of phosphorus (Rivilla et al 2018). These authors have indeed found a positive trend between the abundance of PN and the optically thin 29 SiO isotopologue, which suggests that PN is generated in shocks.…”